1. Field of the Invention
The present invention relates to denture fabrication processes, and more particularly to a process and apparatus for applying thermoplastic impression compound molding onto custom denture impression trays.
2. Description of the Prior Art
The process of fabricating dentures entails a variety of steps each necessary to properly conform the denture interface to the unique muscular and skeletal architecture of the patient. It is a time consuming, elaborate process. In accordance with the currently prevailing practice a customized denture impression tray is first fabricated, generally conforming with the maxillary or mandibular structure of the patient, with the peripheral edges of the tray shortened or reduced to accommodate molding compound impressions of the adjacent anatomical landmarks. These molding compound impressions are made in a border molding bead applied onto the peripheral edges of the tray which is repositioned into the patient while the compound is still soft. The patient is then asked to perform various movements of the facial musculature while the border molding compound is setting to provide in the impression appropriate clearances within the buccal, labial and/or lingual folds. When these borders are thus defined the rest of the tray is filled with impression material to provide a full impression of the denture interface.
In this process the desired border molding compound is one of several thermoplastic compounds having a setting temperature just somewhat higher than the patient""s body temperature. The minimal difference between a tolerable temperature, i.e., temperature at which tissue is injured, and the temperature within the patient""s oral cavity leaves little working range. Within this marginal temperature range the molding compound must be applied as a ribbon or bead along the denture impression tray borders, and thereafter fitted to the patient while still soft. Of course, the bead would frequently set up along the ribbon length and the incidence of improper impression because of already hardened molding compound was both an unwanted and an anticipated event. The practitioner was therefore expected to re-heat the molding compound xe2x80x98stickxe2x80x99 and thereafter parts of the ribbon over an open flame, each time with some concern over excessive temperature levels that may injure the patient. The necessary experience and skill in this border molding process, together with the attention required, have substantially affected the cost of dentures and any simplification techniques would greatly enhance both the quality of the product and its cost. Amongst the simplifications the task directed at maintaining the border molding pliable while it is applied is the most cumbersome and therefore most likely to benefit from any improvement.
In the past various devices have been developed which in one way or another eject heated compounds in the course of dentistry. Examples of such devices are described in U.S. Pat. No. 3,522,654 to Schoelz and U.S. Pat. No. 3,614,389 to Malisza, both directed to electrically heated dental wax dispensers, and U.S. Pat. No. 4,265,618 to Herskovitz et al. describing endodontic syringes for dispensing thermoplastic material. While suitable for the purposes intended, each of the foregoing dispenses thermoplastic matter in quantities that are less than those that can pose substantial risk of burn injury, or that dispense molten substances onto those body elements that are generally resistive to heat.
Concurrent with these developments, those engaged in the adhesives art have also developed a variety of devices that in one manner or another melt matter for use as a glue or adhesive. Examples of such devices include those taught in U.S. Pat. No. 5,026,187 to Belanger et al., U.S. Pat. No. 5,462,206 to Kwasic, U.S. Pat. No. 4,773,566 to Hougland, U.S. Pat. No. 3,744,921 to Weller et al. and others. Again, while wholly suitable for the purposes intended, each of the foregoing dispensers seeks to maximize the best transfer to the thermoplastic material, and considerations of tissue injury are attended primarily by protective structure and shielding.
Also devised in the part are molten adhesive dispensers which include automatic cut-outs, like those taught in U.S. Pat. No. 4,546,235 to Kxc3x6lter and U.S. Pat. No. 6,105,824 to Singleton, and those provided with temperature controllers like those shown in U.S. Pat. No. 3,665,158 to Froedge and U.S. Pat. No. 4,816,642 to Dennison. Each of these, while well suited for its purpose, lacks the requisite control and heat exchange configuration that is required for dispensing matter within the limited temperature range bounded by normal body temperature and the temperatures associated with tissue injury.
Those in the art will appreciate that normal heat transfer exchanges between body temperature to the temperatures of the ambient environment will occur with some expedience, particularly if the surface-to-volume ratio of the heated mass is high. These heat transfer conditions, and the maximum temperature limits of any matter that will contact human tissue, require high volumes of stored molten compound for quick, large quantity dispensing of the border bead. A method and structure adapted to these narrow constraints is extensively sought, and it is one such process and structure that is disclosed herein.
Accordingly, it is the general purpose and object of the present invention to provide a dispenser useful in heating and dispensing thermoplastic molding compound limited in temperature range.
Other objects of the invention are to provide a process for dispensing thermoplastic molding compound in large quantities.
Further objects of the invention are to provide a dispensing structure useful in heating large quantities of thermoplastic molding compound within a limited temperature range.
Yet additional objects of the invention are to provide a method and structure for applying molten border molding material onto the edges of a custom denture impression tray that is limited in temperature range.
Briefly, these and other objects are accomplished within the present invention by providing a thermoplastic molding compound dispenser useful in heating hardened compound in cylindrical cartridges to a limited temperature above melting. More precisely, the dispenser is provided with a cylindrical heating chamber dimensioned for intimate surface contact with the exterior of the cartridge and surrounded by an electrical heating element. This heating chamber is aligned along the longitudinal axis of an elongate dispenser body terminating in a dispensing nozzle at its forward, and a cartridge receiving opening at the rearward end.
Preferably the heating element is positioned close to the forward end of the heating cavity, in series with a temperature control circuit provided with a sensor or thermocouple adjacent the nozzle. A set of radially extending hoops is attached to the body, rearward of the sensor, both to provide finger engagements in the course of dispensing and to serve as sufficiently large heat exchange surfaces for effective temperature control. Furthermore, a generally planar, lateral body extension rearward of the hoops provides the other manipulative structure while also accommodating heat exchange. The presence of such enlarged heat exchange surfaces allows for the closely regulated temperature control accommodating the narrow range of controlled heat ranges of the molding compound.
Preferably, the cavity volume heated by the electric element is sufficient to store the full quantity of the molding compound necessary for the complete border molding ribbon. Concurrently, the surface areas of the hoops and the handle structure selected for sufficient heat loss to overcome any gain due to the heat internally stored, limits any heat excursions beyond those set in the temperature control circuit. In this manner the surface-to-volume characteristics of the dispenser structure are useful in minimizing the potential of an unwanted temperature increase in the molten molding compound while also providing manipulative convenience.
The foregoing dispenser can then be utilized to lay a complete ribbon of molding compound in a single application, extending over the whole of the peripheral edge of the dental casting form which can thereafter be immediately positioned on the maxillary or mandibular surfaces of the patient. Once thus positioned, a well-known sequence of muscular manipulations can then be used to conform the border moldings to a set shape that accommodates the muscular surface distortions. In this manner the process of forming and constructing dentures is rendered convenient, allowing for a much more precise accommodation of the anatomical landmarks and much more comfortable denture.